SceneView GUI - Add "Gizmos" IN Scene (not Overlay)

Hi,

I'm currently working on an Custom SceneView Editor for 3D tile-maps. I need it because I create my full map in Maya and (want to) use this tool to tell Unity where a walkable tile is an where not. It's working greate but I have an annoying probleme. I couldn't figure out a way to render something in the scene and not over the scene (See the picture). I'm currently using Gizmos, because this is the only way I could find to render 3D GUI in the Scene View. Is there another way I'm not aware of?

Second question: Can I deaktivate the standard unity grid and object-selection in my script?

Best Regards, Théo

Here is my Editor code:

PS: I replaced some code with comments to make it easyer for you to read.

PPS: It's intentional that I'm not saving any data in the moment.

 //using ...

 public class TileEditor : Editor {
     static bool running = false;
     static int height = 10;
     static int width = 10;
 
     static List<Vector2> aktiveTiles = new List<Vector2>();
 
     public static void OnSceneGUI(SceneView view)
     {
         Ray ray = HandleUtility.GUIPointToWorldRay (Event.current.mousePosition);
         
         Plane plane = new Plane(new Vector3(0, 1, 0), new Vector3(0, 0, 0));
         float distance;
 
         plane.Raycast(ray, out distance);
 
         Vector3 point = ray.GetPoint(distance);
 
         //Code to check if point is in grid and to add new Vector2 in aktiveTiles
 
         //Code to add IntFields for width & height
 
         for(int x = 0; x < width; x++)
         {
             for(int y = 0; y < height; y++)
             {    
                 //Code to generate Vector[] points
                 Handles.DrawPolyLine(points);
             }
         }
 
         foreach(Vector2 tile in aktiveTiles)
         {
                //Code to generate Vector[] points
             Handles.DrawAAConvexPolygon(points);
         }
     }
 
 
     [MenuItem("Tools/Tile Editor")]
     public static void Trigger()
     {
         running = !running;
         if(running)
             SceneView.onSceneGUIDelegate += OnSceneGUI;
         else
             SceneView.onSceneGUIDelegate -= OnSceneGUI;
         SceneView.RepaintAll();
     }
 }
 

Here is the picture: